1. Field of the Invention
The present invention relates to breakover protection components, currently called Shockley diodes or gateless thyristors, of bidirectional type.
2. Discussion of the Related Art
The diagram of a bidirectional Shockley diode appears in FIG. 1. A first Shockley diode S1 and a second Shockley diode S2 are connected in antiparallel between terminals A1 and A2.
FIG. 2 is a simplified cross-sectional view of a semiconductor component conventionally constituting a bidirectional Shockley diode.
The component is constructed on a very lightly-doped N-type substrate 1 and includes on its upper and lower surface sides P-type regions 3 and 4 occupying the major part of the component surface. Each of regions 3 and 4 contains, in a substantially complementary way, an N-type region, respectively 6 and 7. Substantially facing each of regions 6 and 7, at the interface between the respective P region and the substrate, are disposed lightly-doped N-type regions 8 and 9. The upper and lower circumferences of the substrate are surrounded with a highly-doped N-type ring, respectively 11 and 12. The circumferences of the upper and lower surfaces are coated with insulating layers, currently in silicon oxide, respectively 14 and 15. An upper metallization M1 corresponding to electrode A1 covers regions 3 and 6 and a lower metallization M2 covers regions 4 and 7. These metallizations stop on the peripheral oxide layer, respectively 14, 15.
A first vertical Shockley diode, the anode of which corresponds to the upper surface, includes regions 3-1-9-4-7. A second Shockley diode, the cathode of which corresponds to the upper surface, includes regions 6-3-8-1-4. For each of these diodes, the interface (3-8; 4-9) between the internal P-type region and the corresponding lightly-doped N-type region determines the breakdown voltage.
Generally, the lower surface of the component is intended to be mounted by brazing on a base plate forming a thermal distributor. In the case of the component shown in FIG. 2, this base plate 20 must exhibit a surface smaller than the surface of the component and of lower surface metallization M2. Indeed, when the component is mounted on this plate by a brazing layer 21, the brazing tends to overflow laterally to form pads 22. Should metallization M2 extend on the entire lower surface of the component and plate 20 be wider than the component, the brazing pad would overflow on the lateral surface of the component and short-circuit regions 4 and 7 with substrate 1. The function of the component would no longer be ensured, or would be very degraded.
Thus, a tendency of actual technology, for simplifying the assembly, is to implement so-called well components, that is, components having on their entire circumference a region of the same type of doping as one of the lower surface layers. Thus, a short circuit between the lower surface and the lateral surface no longer hinders the operation of the component.
The transformation of the component of FIG. 2 into a well component is illustrated in FIG. 3. Well 31 is formed by P-type drive-ins extending from lower and upper surfaces and joining lower surface P-type region 4. It should be noted that, in this case, the transition from the simple structure of FIG. 2 to the well structure of FIG. 3 is relatively direct. In particular, the disposition of regions 8 and 9 determining the breakdown voltages of the component is not altered.
As shown in FIG. 3, the component can be mounted on a thermal distributor plate 33 which is wider than the component and a possible brazing pad 35 will not affect the operation of the component since it joins a region of the same type of doping as the useful regions which are desired to be connected by lower surface metallization M2.
FIG. 4 shows a simplified cross-sectional view of another embodiment according to the prior art of a double Shockley diode. The component is again formed based on a substrate 1, including upper and lower P-type regions 3 and 4 wherein are formed upper and lower N-type regions 6 and 7. Lightly-doped N-type regions 8 and 9 at the interface between the P-type regions and the substrate are replaced with lightly-doped N-type lateral regions 41 and 42. This disposition enables reaching of a breakdown voltage lower than that of FIG. 2. Indeed, the breakdown voltage depends on the doping gradient at the junction likely to break down. In the case of FIG. 2, the bottom of a P diffusion, that is, a relatively lightly-doped region is located at this junction. Conversely, in the case of FIG. 4, the upper part of the P diffusion (3 or 4) is in contact with N.sup.- region 41 or 42. As a result, lower breakdown voltages can be obtained, which is desired for some applications.
However, the component of FIG. 4 has the same assembling disadvantages as that shown in FIG. 2.